User equipment (UE), also known as mobile stations, wireless terminals and/or mobile terminals are enabled to communicate wirelessly in a wireless communication system, sometimes also referred to as a cellular radio system. The communication may be made e.g. between two user equipment units, between a user equipment and a regular telephone and/or between a user equipment and a server via a Radio Access Network (RAN) and possibly one or more core networks.
The user equipment units may further be referred to as mobile telephones, cellular telephones, laptops with wireless capability. The user equipment units in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the radio access network, with another entity, such as another user equipment or a server.
The wireless communication system covers a geographical area which is divided into cell areas, with each cell area being served by a network node, or base station e.g. a Radio Base Station (RBS), which in some networks may be referred to as “eNB”, “eNodeB”, “NodeB” or “B node”, depending on the technology and terminology used. The network nodes may be of different classes such as e.g. macro eNodeB, home eNodeB or pico base station, based on transmission power and thereby also cell size. A cell is the geographical area where radio coverage is provided by the network node/radio base station at a base station site. One radio base station, situated on the base station site, may serve at least one cell. The network nodes communicate over the air interface operating on radio frequencies with the user equipment units within range of the respective network node.
In some radio access networks, several network nodes may be connected, e.g. by landlines or microwave, to a Radio Network Controller (RNC) e.g. in Universal Mobile Telecommunications System (UMTS). The RNC, also sometimes termed a Base Station Controller (BSC) e.g. in GSM, may supervise and coordinate various activities of the plural network nodes connected thereto. GSM is an abbreviation for Global System for Mobile Communications (originally: Groupe Spécial Mobile).
In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), network nodes, or radio base stations, which may be referred to as eNodeBs or even eNBs, may be connected to a gateway e.g. a radio access gateway, to one or more core networks.
UMTS is a third generation mobile communication system, which evolved from the GSM, and is intended to provide improved mobile communication services based on Wideband Code Division Multiple Access (WCDMA) technology. UMTS Terrestrial Radio Access Network (UTRAN) is essentially a radio access network using wideband code division multiple access for user equipment units. The 3GPP has undertaken to evolve further the UTRAN and GSM based radio access network technologies.
The 3GPP is responsible for the standardization of GSM, UMTS, LTE and LTE-Advanced. LTE is a technology for realizing high-speed packet-based communication that may reach high data rates both in the downlink and in the uplink, and is thought of as a next generation mobile communication system relative UMTS.
In the present context, the expressions downlink, downstream link or forward link may be used for the transmission path from the network node to the user equipment. The expression uplink, upstream link or reverse link may be used for the transmission path in the opposite direction i.e. from the user equipment to the network node.
A basic principle used in several different cellular wireless communication systems (GSM, UMTS, LTE) comprises the following steps:                (a) each network node sends a pilot signal,        (b) the user equipment measures the received pilot signal strength from the serving network node and from several neighbouring network nodes,        (c) the user equipment reports back to the serving network node the result of the measurements, and        (d) the serving network node decides whether the user equipment should be handed over to another cell or not.        
Typically, the user equipment is instructed to connect to the network node with the highest received pilot power, which defines the reference cell size. In LTE, the procedure is referred to a Reference Signal Received Power (RSRP) based cell selection.
However one potential solution for achieving higher data rates and network capacity with LTE-Advanced is the deployment of Heterogeneous Networks (HetNet). In HetNet deployments the original Macro-cell layout is complemented with Low Power Nodes (LPN), such as micro nodes, pico nodes, or relay stations (RS). Relay stations are considered as an LTE-Advanced tool for mainly improving/extending the Macro base station coverage and providing higher cell-edge throughput.
The relay station is connected to the donor cell, herein referred to as Radio Base Station (RBS), which may comprise e.g. a macro base station, a micro base station or a pico base station, via a wireless backhaul link. The wireless backhaul link may operate either in the same frequency (inband) or in a different one (outband) with respect to the frequency used in the links between the relay station and the user equipment and between the radio base station and the user equipment, respectively. Thus, in a relay deployment there are three different types of links, namely the links between the radio base station and the user equipment, which may be referred to as direct links. Other kinds of links comprise the links between the relay station and the user equipment, also referred to as access links. Further, there may also be links between the radio base station and the relay station, called backhaul links. For inband relay stations, sharing of the available resources between the access links and the backhaul links imposes limitations on the improvement of the experienced user throughput.
Deployment of relay stations may be based on a radio base station coverage criterion. Relay stations may be placed in areas which are either out of radio base station coverage or exhibit low user equipment bit rates. Relay stations may be equipped with omni-type of antennas thus providing coverage to both indoor and outdoor areas that surround them. In addition to the relay antennas beam shape, the active links between user equipments and nodes (radio base station or relay station) are formed based on the existing (baseline) handover strategy which is inherited from the pure radio base station deployments. Specifically, each user equipment measures the downlink received signal strength, Reference Signal Received Power (RSRP), from a number of network nodes (radio base stations or relay stations) which are in the neighbourhood of the user equipment. Then the user equipment connects to the network node (radio base stations or relay stations) that provides the highest RSRP value.
The RSRP-based cell selection may be acceptable also on the uplink, under some circumstances. However, if there is a difference between the pilot power sent by two network nodes, then the RSRP from the high-power network node may be stronger than the RSRP from the low-power network node, although the user equipment is closer to the low-power network node from a radio and geographic point of view, as the case may be in heterogeneous networks. The coverage of the low-power network node may be significantly smaller than the coverage of the high power network node.
There is a need for improvements in conjunction with handover decisions based on RSRP measurements as a radio base station cell, which may comprise a macro base station, a micro base station, or a pico base station, is complemented with relay stations.